Squirrel monkeys (Saimiri sciurea) have been implanted subcutaneously with 250 mg diethylstilbestrol (DES), estradiol (E2) or remain untreated. The purpose of this study is to analyze the relationship between plasma levels of estrogen (monitored monthly), rate and extent of development of pelvic mesotheliomas induced by estrogen exposure, and the concentration and biochemical characteristics of cytosol estrogen binding protein in tumor tissue. In addition induced tumors are to be characterized histologically and ultrastructurally. A radioimmunoassay is under development for measurement of plasma DES levels. Study of metabolism of DES after injection with labelled tracer suggests that most of the DES is excreted unchanged in the feces.